While internal combustion engines on automobiles or the like are in operation, it has been customary from the standpoint of exhaust gas purification to add an air-fuel ratio sensor and control the amount of a supplied fuel for maintaining a good air-fuel ratio according to an air-fuel ratio feedback control process.
Even if an exhaust gas itself is kept in a favorable condition, however, variation in the air-fuel ratios in different cylinders tend to lower the purification ratio of a catalyst which purifies the exhaust gas. Accordingly, efforts have been made to monitor variation in the air-fuel ratios in respective cylinders {see Japanese Laid-Open Patent Publication No. 2009-270543 (hereinafter referred to as “JP2009-270543A”) and Japanese Laid-Open Patent Publication No. 03-189371 (hereinafter referred to as “JP03-189371A”)}.
According to JP2009-270543A, a cyclic period in which the combustion of fuel is cycled once through all the cylinders is regarded as one cycle, and air-fuel ratio fluctuations in one cycle are measured. If the measured air-fuel ratio fluctuations are too large, the internal combustion engine is judged as malfunctioning (see Abstract).
According to JP03-189371A, when a misfire is detected, ignition control processes for the respective cylinders are individually stopped successively, and an average output value of the air-fuel ratio sensor and output amplitude values thereof at this time are read and compared with the corresponding values read prior to stoppage of the ignition control processes, thereby identifying a misfiring cylinder (see claim 1). It is stated in the publication that the internal combustion engine may have one air-fuel ratio sensor or a plurality of air-fuel ratio sensors (page 4, lower left column, lines 6 through 10).